London -LRB- CNN -RRB- A UK public inquiry into the 2006 death of Russian spy Alexander Litvinenko opened Tuesday at the Royal Courts of Justice in London , after years of wrangling over what evidence can be heard .

In a deathbed statement , Litvinenko blamed Russian President Vladimir Putin for ordering his poisoning by tea at a London hotel . The Kremlin has always strongly denied the accusation .

Sir Robert Owen , who 's chairman of the inquiry , said Tuesday that sensitive material relating to possible Russian state involvement in Litvinenko 's death would be heard behind closed doors .

The British government initially rejected requests to hold a public inquiry , but the decision was reversed last summer after Litvinenko 's widow , Marina Litvinenko , challenged it in court .

She argued that a public inquiry would enable the fullest possible investigation .

An inquest -- a coroner-led investigation that is held as a matter of course in the case of unnatural deaths in England -- had been opened after her husband 's death . But unlike a public inquiry , it can not hear evidence behind closed doors .

In Alexander Litvinenko 's case , such evidence could involve matters of national security .

`` The issues to which his death gives rise are of the utmost gravity and have attracted worldwide interest and concern , '' Owen said .

Litvinenko , a former KGB agent and fierce critic of Putin , came to Britain in 2000 after turning whistle-blower on the FSB , the KGB 's successor .

He died at a London hospital on November 23 , 2006 , after being poisoned by the radioactive material polonium-210 while drinking tea at the Millennium Hotel in London 's Grosvenor Square .

UK prosecutors have asked for the extradition of two men , Andrei Lugovoi and Dmitry Kovtun , from Russia in connection with Litvinenko 's murder .

But Moscow has refused , saying Russia 's constitution does not allow the extradition of Russian citizens . Both men deny involvement in Litvinenko 's death .

Owen said that Lugovoi and Kovtun had been invited to give evidence to the inquiry by video link from Russia and that he hoped they would do so .

The public inquiry will look at possible Russian state involvement in Litvinenko 's death . However , it will not address the question as to whether the UK government could , or should , have taken steps to prevent the murder .

Litvinenko is said by his widow to have been a British agent , with a handler at MI6 , Britain 's foreign security service .

In 2012 , the counsel to the inquest , Hugh Davies , said evidence provided by the UK government showed Russian involvement and `` does establish a prima facie case as to the culpability of the Russian state in the death of Alexander Litvinenko . ''

The inquest has been put on hold while the public inquiry is held .

Owen , the coroner in the inquest , said the open hearings in the inquiry should conclude before Easter ; that is , early April .

More than 70 witnesses are due to be called over the coming weeks , including family and friends of Litvinenko , those who worked with him before his death , medical staff who treated him after he fell ill and the pathologists who conducted his autopsy , the court heard .

The pathologists will testify Wednesday on the postmortem results . The presence of radiation in Litvinenko 's body complicated the autopsy , the inquiry heard .

A nuclear scientist will also give evidence Wednesday about polonium-210 , its qualities , where it can be found and what effect it has on the body once ingested .

In the course of the inquiry , evidence may also deal with the contamination risk posed to the wider public by the transfer of such highly radioactive material .

Owen said polonium could have been used to `` kill large numbers of people or spread general panic and hysteria among the public . ''

@highlight

A public inquiry into the former KGB agent 's poisoning death opens in London

@highlight

Alexander Litvinenko died in 2006 after being poisoned by radioactive material

@highlight

He blamed Russian President Vladimir Putin ; the Kremlin denies involvement